Vision depends on the accurate processing of the image by the neural retina. The development of the retina involves a series of events that result in the proper wiring of its circuitry. The first step in building retinal circuits is the generation of each major class of retinal neuron and subsequently, establishing the appropriate connectivity between the cell components. Our understanding of the mechanisms that regulate these major steps in development is not yet complete. In particular, it has not been possible to determine the cellular processes involved in dynamically shaping the number, location and wiring patterns of retinal neurons throughout their entire time-course of development in vivo. This knowledge is important as the consequences of disrupting cellular function during development is more readily identified when we have a real-time view of the developmental events as they occur in the live animal. It is now possible to follow the development of retinal neurons from the time of their genesis to when they form circuits in the rapidly developing and largely transparent zebrafish embryo. In this proposal, we will focus on a major cell component of the outer retina, the horizontal cells, and determine: (Aim 1) how an unconventional mode of neurogenesis produces the right number of horizontal cells, and how migration of the daughter cells lead to the formation of their mosaic-like network, and (Aim 2) determine what cell-cell interactions, involving neurons and Muller glial cells, are important for establishing horizontal cell connectivity with cone photoreceptors. Our approach is to use time-lapse multiphoton imaging of fluorescently labeled retinal neurons to track their behavior over hours to days, and a targeted cell ablation approach using the titanium:sapphire infrared laser to unravel cell-cell interactions. Together, the proposed in vivo studies will increase our understanding of the cellular behaviors and interactions necessary for establishing normal retinal networks and provide the foundation for future investigations on retinal disease and injury.